xtensa: fix access check in csum_and_copy_from_user

Commit d341659f ("xtensa: switch to providing
csum_and_copy_from_user()") introduced access check, but incorrectly
tested dst instead of src.
Fix access_ok argument in csum_and_copy_from_user.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Fixes: d341659f ("xtensa: switch to providing csum_and_copy_from_user()")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>